Genetic and biochemical studies are being carried out on tumor viruses, SV40 and MSV, and on the mitochondrial-specific and virus- induced dT kinases. Conditional lethal mutants of SV40 are being screened to obtain "early" mutants and host-dependent mutants. A putative helper-independent MSV strain and a cold-sensitive MSV mutant are being characterized. Also under investigation are the control of synthesis of viral-induced cytosol dT kinases and of mitochondrial- specific dT kinase.